The present invention relates to hydrocarbon oxidation catalysts and catalyst preparation and especially to a hydrocarbon oxidation process for producing maleic anhydride using a catalyst prepared in a certain manner and containing vanadium, phosphorus and silicon oxides.
The use of catalysts containing vanadium, phosphorus and silicon oxides for oxidation of hydrocarbons to maleic anhydride is disclosed in a general fashion in many prior art references. For example, U.S. Pat. No. 2,778,838 to Reid et al discloses the use of various supports such as kieselguhr (mostly silica) for a vanadium-phosphorus oxide catalyst used in the production of maleic anhydride. According to the Reid et al patent, the support is an inert carrier for the vanadium and phosphorus oxides.
U.S. Pat. Nos. 3,156,705, 3,156,706, 3,156,707, 3,238,254, 3,255,211, 3,255,212, 3,255,213, 3,288,721, 3,351,565, and 3,385,796 to Kerr also disclose in a general fashion the use of various materials, including silica, as a support for a vanadium-phosphorus oxide catalyst used in producing maleic anhydride, in particular from an unsaturated hydrocarbon feed such as butene. These patents to Kerr usually mention the use of silica materials as a support and/or as inert diluents and not as part of the catalyst material coprecipitated with vanadium and phosphorus oxides. Apparently all the patents to Kerr state (see, for example, Col. 5, lines 32-39 of U.S. Pat. No. 3,156,707) substantially as follows:
"Inert diluents such as silica may be present in the catalyst, but the combined weight of the essential ingredients phosphorus, oxygen and vanadium should preferably consist essentially of at least about 50 weight percent of the composition which is coated on the carrier, if any, and preferably these components are at least about 75 percent of the composition coated on the carrier, and more preferably at least 95 weight percent."
U.S. Pat. No. 3,226,337 to Riemenschneider et al also mentions the use of silica as an inert carrier at Col. 1, line 46, and Col. 2, line 43. U.S. Pat. No. 3,293,268 to Bergman et al mentions the use of silicon carbide as an inert carrier at Col. 3, line 60. Also, U.S. Pat. No. 3,759,848 to Felice mentions the use of silica as an inert support.
Patents outside the maleic anhydride production area but of interest regarding the disclosure of the use of silica in catalysts include U.S. Pat. Nos. 1,463,206, 1,852,207 and 2,086,542, all of which are early patents relating to the use of vanadium oxide on a support, for example a siliceous support, as an oxidation catalyst.
U.S. Pat. Nos. 2,120,702, 2,275,182, 2,575,457, 2,613,187, 2,694,686, 3,132,109, 3,213,035 and 3,673,111 disclose the use of phosphoric acid on a support, such as a silica support, as olefin polymerization catalysts. The catalysts are typically produced by impregnating phosphoric acid onto a porous support and calcining, but the catalysts can be produced by alternate methods. For example, in U.S. Pat. No. 2,613,187, a catalyst is prepared by mixing pyrophosphoric acid, diatomaceous earth, and a polymeric dimethyl siloxane to form a composite, and calcining said composite.
U.S. Pat. Nos. 3,243,385 and 3,480,564 relate to vanadium-containing ammoxidation catalysts useful for oxidation of olefins to acrylonitrile. The catalyst of U.S. Pat. No. 3,243,385, for example, can be prepared using ammonium metavanadate, phosphoric acid, colloidal silicic acid and tin nitrate.
U.S. Pat. No. 3,351,566 discloses stabilization of a nickel surface area of a nickel-silica catalyst wherein the catalyst is prepared by coprecipitating a nickel cation with a silicate anion onto a porous solid silica.
U.S. Pat. Nos. 3,183,196 and 2,738,336 are of special interest with respect to the present invention. U.S. Pat. No. 3,183,196 discloses a catalyst for oxidation of hydrocarbons such as oxidizing orthoxylene to phthalic anhydride, benzene to maleic anhydride, and anthracene to anthraquinone. The catalyst of the '196 patent is prepared by producing a hydrate of silica (for example, by kneading the silica in water), adding a vanadium compound to said hydrate to obtain a pasty mixture, and adding any other components desired and then calcining.
U.S. Pat. No. 2,738,336 is particularly directed to preparing an olefin polymerization catalyst somewhat similar to the olefin polymerization catalysts of the patents previously cited. However, instead of the typical impregnation of phosphoric acid onto a support, according to the '336 patent disclosure the catalyst is made by mixing a polyphosphoric acid and an alkyl orthosilicate and then calcining the composite. According to to the date discussed in the patent at Col. 5, using the alkyl orthosilicate results in a better silica-containing catalyst for propylene polymerization than when the catalyst is made using kieselguhr.